Attention is directed to our co-pending patent application, filed herewith, entitled "Wire-Laying Machine for Medical Imaging Devices", hereby incorporated by references.
The development of surgical and medical treatments for internal disorders, such as heart disease, has dramatized the need for non-invasive methods of diagnosing such conditions. For example, up until recently ischemic heart disease, a common adult cardiac problem caused by inadequate blood flow to regions of the heart, could only be diagnosed by a coronary arteriogram. Since the arteriogram involved an injection directly into the heart chamber via a catheter, this risky procedure was only warranted when the heart condition became severe (i.e. chronic chest pain--angina pectoris, circulatory failure--congestive failure or infarction--heart attack). Aside from the fact that catheterization was unacceptable as a screening technique, it was also not possible to make comparitive measurements of resting and stressed conditions.
Recently, radionuclides have been developed that permit heart imaging without catheterization, and which also permit measurement directly after exercise. Radioactive potassium-43 and potassium-analogues, such as rubidium-81,cesium-129 and thallium-201, have been proposed for intra-venous injection and measurement. Once injected into a vein these compounds travel to the heart and reach a maximum distribution (heart-to-blood) within a few minutes. At present, thallium-201, an isotope with a three-day half-life, is in use as a radiotracer because its decay products may be imaged with conventional scintillation cameras. However, its relatively long half-live and high patient dose often preclude serial examinations. In heart diagnostic use a scintillation camera image is compared to a normal or expected radioisotope distribution; areas of low radioisotope perfusion suggest dead myocardial cells due to an infarction or other blood flow restrictions.
Prior art scintillation cameras operate on the principle that a high energy photon (gamma ray) emitted from a radioisotope will produce a flash of light in a phosphorescent crystal, such as sodium iodide. Typically, a camera will employ a large, thin scintillation crystal, an array of photomultiplier tubes, a multichannel collimator, so that only the radioactivity directly in front of the sensitive surface can register, and circuitry to analyze the pulses produced by the photomultipliers to create a distribution image of the radiotracer in the patient.
The scintillation camera suffers from a number of limitations on its ability to accurately assess disorders in myocardial perfusion. Firstly, the resolution of a scintillation camera varies with depth. Secondly, the gamma radiation is attenuated in tissues between the region of interest and the detector. Thirdly, when a scintillation camera is used with very short half-life, higher activity radiotracers, such as rubidium 82 (a positron emitter with a 75 second half-life), the scintillation camera saturates below the activity levels necessary for adequate statistics, and, fourthly, the sensitivity of the camera is 1imited by the "solid angle" subtended by a collimator hole.
Moreover, conventional scintillation cameras fail to make use of the unique characteristics of positron-emitting isotopes. When a position is emitted during radioisotope decay, the positron is quickly annihilated by collision with an electron; the annihilation generates two collinear (back-to-back) 511 keV gamma rays. Thus, two detectors with sufficiently large surface areas positioned on opposite sides of the patient can detect the time coincident gamma rays and thereby determine their flight path. When sufficiently large statistics are available a lateral tomograph of the organ of interest can be obtained and the sensitivity is now determined by the solid angle subtended by the whole camera as opposed to a collimator hole.
Various attempts have been made to develop an apparatus to detect the collinear gama rays generated by positron annihilation. Scintillation techniques have been less than successful because of the limitations on spatial resolution inherent in the crystals. Attention is directed to an article by J.E. Bateman et al entitled "The Development of the Rutherford Laboratory MWPC Positron Camera" in 176 Nuclear Instruments and Methods, 83-88 (1980); an article by A. Jeavons and K. Kull entitled "A Proportional Chamber Positron Camera for Medical Imaging" in 176 Nuclear Instruments and Methods, 89-97 (1980); an article by A. Jeavon and G. Charpak entitled "The High Density Multiwire Drift Chamber" in 124 Nuclear Instruments and Methods, 491-503 (1975); and an article by A. Jeavon and C. Cate entitled "The Proportional Chamber Gamma Camera" published in the Proceedings of the IEEE Nuclear Science Symposium, 19-21 (1975).
Other examples of radiation imaging systems, in general, may also be found in the following: U.S. Pat. No. 3,359,421, issued to V. Perez-Mendez et al on Dec. 19, 1967, entitled "Magnetostrictive Readout For Wire Spark Chambers"; U.S. Pat. No. 3,703,638, issued to R. Allemand et al on Nov. 21, 1972, entitled "Ionization Radiation Detector System For Determining Position Of The Radiation"; U.S. Pat. No. 3,772,521, issued to V. Perez-Mendez on Nov. 13, 1973, entitled "Radiation Camera And Delay Line Readout"; U.S. Pat. No. 3,786,270, issued to C. Borkowski et al on Jan. 15, 1974, entitled "Proportional Counter Radiation Camera".
Typically, non-scintillating, radiation imaging cameras employ a conversion medium (wherein the high energy gamma ray is absorbed, displacing an electron), an electron amplifier, a spatial detection chamber (to determine the position of the amplified electrons), and circuitry to analyze the electrons detected by the proportional chamber and produce an image.
Essentially, a spatial detection chamber permits accurate location of a ion or electron by detecting the electric field disturbances it causes in a charged cartesian coordinate grid. Typically, a series of parallel anode wires are overlaid perpendicular to a series of parallel cathode wires. A charged particle entering the system is drawn to the nearest oppositely charged wire, causing a field disturbance (avalanche) and, consequently, an electrode pulse on the nearest anode and cathode wires which can be detected with appropriate circuitry to define the impact point in the X-Y coordinate plane of the chamber. See B. Rossi and H. Staub, Ionization Chambers and Counters (McGraw Hill Publishing Co. 1969) for further details on proportional counters.
Various problems have been encountered by researchers attempting to develop a practical positron imaging camera. Selection of a conversion medium is crucial. The medium must stop a high percentage of gamma rays and convert them into Compton electrons or photoelectrons yet the material must be porous so that the freed electrons leave the solid and enter the gas. Additionally, since the detector circuitry (i.e. the proportional chamber) must be separate from the conversion medium, a means is needed for transferring and amplifying the conversion events so they may be read by the proportional chamber. Moreover, the circuitry from the chamber must be able to handle high data rates on the order of 100,000 counts per second and obtain good resolution (.about. 2 mm). In sum, there exists a need for an inexpensive, efficient radiation imaging camera, robust enough for use in a hospital setting and accurate enough to permit computer-generated tomographs of internal organs using positron-emitting radioisotopes.
In connection with the need for tomographic projections, it should be noted that even when two cameras are used to detect collinear gamma rays from a single annihilation, and both photons are detected, one only knows that the source must lie on a line formed by the two conversion points. To produce a tomograph it is necessary to determine where the source lies along the line. This requires that a positron camera accept events with photon trajectories over a spread in angles to obtain a stereoscopic view of the organ. Therefore, there also exists a need for a first-order focusing mechanism which can be incorporated into the electronics of the gamma ray imaging camera.